Process for the production of molded bodies from silicon-infiltrated, reaction-bonded silicon carbide

Abstract

Silicon-infiltrated, reaction-bonded silicon carbide molded bodies for application in machine building are produced by means of a siliconizing apparatus, comprising a graphite plate coated with boron nitride, a porous silicon carbide plate coated with a layer of boron nitride, silicon carbide and carbon and a layer of lumpy metallic silicon inserted between the two plates. Furthermore, following the heating and solidification of the liquid silicon, wherein due to the increase in volume of the silicon, beads of silicon appear at the surface of the molded body, these may be easily removed when the infiltrated silicon carbide molded bodies are cooled in a nitrogen atmosphere.

What is claimed is: 
     
       1. A process for the siliconizing of porous molded silicon carbide bodies, comprising the steps of: (a) placing a molded silicon carbide body which contains elemental carbon onto an apparatus comprising a base plate comprising a graphite plate coated with boron nitride and a support plate, comprising a porous plate comprising silicon carbide which is infiltrated with silicon, and a coating which comprises a mixture of boron nitride, silicon carbide and carbon, and a layer comprising silicon therebetween;   (b) heating the molded silicon carbide body and support and base plates under vacuum to a temperature higher than the melting point of elemental silicon, for a holding time of from about 0.5 to 6 hours, thereby infiltrating the silicon carbide molded body with silicon; and   (c) thereafter cooling the molded body in a nitrogen atmosphere under a pressure of from about 0.05 to 1 bar.   
     
     
       2. A process according to claim 1, wherein the support plate comprises an originally porous silicon carbide plate into which metallic silicon has been infiltrated and the coating comprises a mixture of about 2 parts by weight carbon, about 3 parts by weight boron nitride and about 5 parts by weight silicon carbide, and further comprising a mixture of silicon carbide and carbon granules scattered thereon. 
     
     
       3. A process according to claim 1, wherein the support plate is made by molding a mixture of silicon and silicon carbide in the presence of carbon. 
     
     
       4. A process according to claim 1, wherein the layer between said support and base plates comprises lumpy metallic silicon, or granules comprising silicon carbide and carbon. 
     
     
       5. A process according to claim 1, wherein the porous molded body comprises a mixture of silicon carbide and carbon. 
     
     
       6. A process according to claim 1, wherein the heating step comprises heating the molded body to a temperature in excess of about 1410° C. 
     
     
       7. A process according to claim 2, wherein said base plate is produced by coating a fine grained graphite plate with a suspension of boron nitride powder in water, and then drying said coated plate. 
     
     
       8. A process according to claim 1, wherein said support plate is produced by the steps of: (a) impregnating a porous support plate of silicon carbide with silicon;   (b) furnace cooling the impregnated plate;   (c) cleaning and smoothing the plate;   (d) coating the plate with a suspension in alcohol comprising about 2 parts by weight carbon, about 3 parts by weight boron nitride, and about 5 parts by weight silicon carbide;   (e) drying the plate; and   (f) scattering granules of a mixture of silicon carbide and carbon having a grain size of from about 0.5 to 1 mm onto the coating.   
     
     
       9. A process according to claim 4, wherein the lumpy silicon metal has a grain size of from about 3 to 10 mm. 
     
     
       10. A process according to claim 1, wherein the molded body is heated at a rate of about 300° C./h to about 1600° C. under a vacuum of about 1 Torr. 
     
     
       11. An apparatus for the siliconization of porous molded silicon carbide bodies, comprising a base plate and a support plate, wherein the base plate comprises a graphite plate coated with boron nitride, and the support plate comprises a silicon-impregnated, porous silicon carbide support with a layer comprising carbon, boron nitride, and silicon carbide applied thereto wherein the support plate is stacked on top of the base plate, and a layer comprising metallic silicon is placed between the two plates.